**4.2 Generation of new faults in intact rock at low shear stresses nearby a preexisting fault caused by the fan mechanism**

This section proposes an alternative explanation to the fact that earthquakes are commonly attributed to pre-existing faults. Pre-existing discontinuities play the role of local stress concentrators, creating the starting conditions for the fanstructure formation. After completion of the initial fan structure, it can create a new dynamic fault in the form of earthquake by propagation through intact rock mass loaded by low shear stresses. **Figure 15** illustrates one of the many models for generation of high local stress on the basis of pre-existing fault. It shows a rock fragment involving a pre-existing fault (black line) with a compressive jog. This

#### **Figure 15.**

*Features of generation of a new extreme rupture in pristine hard rock in the vicinity of a pre-existing fault at low field shear stresses caused by the fan mechanism.*

*Earth Crust*

**Figure 14.**

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the completed block rotation.

are subjected to significant irreversible deformation. **Figure 13b** and **c** demonstrates that in brittle rocks the irreversible deformation in jogs is associated with formation of a row of domino blocks (modified photograph from [29]). The general fault here consists of a number of segments represented by primary ruptures. The propagation of

*The fan mechanism is predominantly activated in fault segments of lower hierarchical ranks (schematic* 

*illustration) providing extreme dynamics along thin localised zones.*

The domino structure of the next hierarchical ranks can also be involved in the fanstructure formation. This feature is illustrated in **Figure 14**. It was observed in [38] that segmentation as a mechanism of fault propagation acts on all hierarchical ranks of complex faults. Once a number of segments of a given hierarchical rank coalesce, they behave as a whole as a new and longer segment of one higher rank. Segment of higher rank can trigger a new segment (shear fracture) at greater distance. A photograph in **Figure 14a** (modified from [29]) shows a fault fragment involving segments of three hierarchical ranks. The structure of this fault is shown symbolically on the left. It incorporates primary ruptures and higher rank segments formed on the basis of compressive jogs represented by the domino structure (rank II and rank III). Domino blocks involved in segments of higher rank can form the fan structure similar to primary ruptures due to rotation of them caused by shear displacement of the rupture faces. However, the complete fan structure can be formed if shear displacement between the fault faces dfault is sufficient for

**Figure 14b** shows the initial and final positions of domino blocks for two shear ruptures of thicknesses h1 and h2. The thick rupture requires significantly

primary ruptures in hard rocks at high σ3 is governed by the fan mechanism.

fragment is located at great depth where the minor stress σ3 is high enough for the fan-mechanism activation in intact rock. Horizontal lines on the graph below indicate symbolically levels of the following parameters: τs is the strength of intact rock, τf is the frictional strength of pre-existing fault, τfan is the transient strength of intact rock determined by the fan mechanism, τ0 is the field shear stress applied to the rock fragment, τ1 is the field stress after the rupture propagation and ∆τ is the stress drop. Orientation of the field shear stress is shown by open arrows.

Because the level of field stress τ0 is significantly less than the frictional strength τf, the situation on the pre-existing fault is very stable. However, due to deformations along the fault caused by shear stresses τ0, a high local stress can be created in the jog zone delineated by a red circle. If the local stress in intact rock of this zone reaches the level of rupture strength τs, the fan structure can be formed. After formation of the fan structure, it can propagate spontaneously through intact rock mass at low shear stresses τ<sup>0</sup> in accordance with Class III behaviour discussed in **Figure 3** and generate an earthquake. The new fault is shown by a white line, and the propagating fan head is represented by red ellipsis. Due to very high brittleness of this rock associated with extremely low rupture energy provided by the fan mechanism, the failure process can be accompanied by abnormal energy release and violence. It should be emphasised that despite the fact that the new fault is formed in intact rock, the magnitude of stress drop ∆τ can be very low because this process takes place at low shear stress applied. The stress drop can be even less than at the stick-slip process in the case of activation of the pre-existing fault.

Thus, the fan mechanism favours the generation of new faults in hard intact rock mass adjoining a pre-existing fault in preference to frictional stick-slip instability along the pre-existing fault. Each earthquake generated by the fan mechanism is associated with formation of a new fault at a new location in the vicinity of a preexisting fault. Furthermore, each new fault can serve as a stress concentrator for generation of the next new fault. The proximity of the pre-existing fault to the zone of dynamic new fracture development in intact rock creates the illusion of frictional stick-slip instability of the pre-existing fault, thus concealing the real situation.

At the same time, there are many evidences that earthquakes are associated with the formation of new faults in the proximity of pre-existing faults. For example, **Figure 16** shows maps of earthquakes in a New Zealand region of relative motions between the Australian and Pacific plates which are not accommodated on one general fault, but on many faults across a wide zone. **Figure 16a** (from [40]) shows

#### **Figure 16.**

*Maps of spatial distribution of earthquake hypocentres and faults on the earth's surface for a New Zealand region [40, 41].*

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**Figure 17.**

*earthquake frequency.*

*Dramatic Weakening and Embrittlement of Intact Hard Rocks in the Earth's Crust at Seismic…*

spatial distribution of hypocentres at depths <40 km. Some earthquake faults generated at great depths can reach the earth's surface as shown in **Figure 16b** (from [41]). The formation of each fault from reaching the earth's surface is associated with an earthquake. The fan mechanism which makes intact hard rocks weaker in respect of dynamic shear rupturing than pre-existing faults is responsible for the spatial distribution of earthquake hypocentres and for the fact that the earth's crust is riddled with faults. A series of aftershocks, which usually accompany the main act of an earthquake, can also be explained by the formation of a series of new faults, where each new fault creates the conditions for activating the fan mechanism

**4.3 Depth distribution of rock strength, brittleness and earthquake activity** 

**Figure 17d** shows a typical histogram of depth distribution of earthquake frequency (from [7]). It demonstrates that earthquake activity varies with depth and has a maximum at a certain depth. Today there are two fundamentally different explanations for this earthquake feature. Both of them consider earthquakes as stick-slip instability on pre-existing faults. The first one is based on the fact that the frictional strength (determining the lithospheric strength) in the upper crust increases with depth in accordance with Byerlee's friction law [10], while in the lower crust it decreases accordingly to a high-temperature steady-state flow law [2, 7]. The second one is based on the velocity-weakening and velocity-strengthening concept [7, 9]. We introduce here a new concept which is based on the new understanding about (unknown before) properties of hard rocks at seismic depth's caused by the

**Figure 17a**–**c** shows symbolically depth distribution of the fan-mechanism efficiency, rock strength profiles and rock brittleness. These graphs are analogous to the dependencies discussed in **Figures 11** and **12**. The fan mechanism can operate at depths where temperature (rising with depths) does not prevent the fan-hinged shear. The new strength profile for hard rocks in **Figure 17b** shows that at low depths corresponding to σ3 < σ3fan(min), the lithospheric strength is determined solely by frictional strength τf. At greater depths corresponding to the range of the fan-mechanism activity, the situation is specific. In the absence of conditions for

*Relation between depth distribution of the fan-mechanism efficiency, rock strength, rock brittleness and* 

*DOI: http://dx.doi.org/10.5772/intechopen.85413*

in the adjacent zone of intact rock.

fan mechanism [20, 23, 42].

**caused by the fan mechanism**

*Dramatic Weakening and Embrittlement of Intact Hard Rocks in the Earth's Crust at Seismic… DOI: http://dx.doi.org/10.5772/intechopen.85413*

spatial distribution of hypocentres at depths <40 km. Some earthquake faults generated at great depths can reach the earth's surface as shown in **Figure 16b** (from [41]). The formation of each fault from reaching the earth's surface is associated with an earthquake. The fan mechanism which makes intact hard rocks weaker in respect of dynamic shear rupturing than pre-existing faults is responsible for the spatial distribution of earthquake hypocentres and for the fact that the earth's crust is riddled with faults. A series of aftershocks, which usually accompany the main act of an earthquake, can also be explained by the formation of a series of new faults, where each new fault creates the conditions for activating the fan mechanism in the adjacent zone of intact rock.
